The operation of some typical microelectronic image projection devices (MIPDs) is based upon the optical properties of liquid crystal in the presence or absence of an electric field. The orientation of the liquid crystal, which may be varied with an applied electric field, is used to modulate reflected light. The use of liquid crystal leads to several fabrication considerations, including containment of the liquid crystal.
FIGS. 1A and 1B illustrate a side view and top view, respectively, of a MIPD using liquid crystal in accordance with the prior art. MIPD 100 as shown in FIG. 1A includes a substrate 105 having an active area 106. The active area 106 comprises the pixels of the MIPD 100. The active area 106 is covered by liquid crystal 110. MIPD 100 also includes a cover glass 115 that seals the liquid crystal 110 from the environment and prevents the liquid crystal from spilling. The liquid crystal is encapsulated between the substrate 105 and the cover glass 115. The device operates by varying the electric potential between the pixels of active area 106 and the cover glass 115. Light incident upon the cover glass 115 is modulated and reflected by the liquid crystal 110. The modulation depends upon the orientation of the liquid crystal molecules, which can be adjusted by varying the potential between the cover glass 115 and the pixels of active area 106.
As shown in the top view of FIG. 1B, MIPD 100 also includes glass (silica) beads 111 randomly dispersed across the surface of substrate 105 covered by cover glass 115. The glass beads 111 are introduced to keep the cover glass 115, which is quite thin, from sagging and thereby maintain the distance between cover glass 115 and active area 106. Typically glass beads 111 are randomly sprayed between the cover glass 115 and the active area 106.
In order to keep the cover glass 115 fixed atop the liquid crystal 110 and to prevent the liquid crystal from spreading beyond the active area 106, an epoxy barrier 120 is typically used. The epoxy barrier 120 is typically deposited as a bead along the perimeter of the active area 106.
The typical fabrication method of a MIPD has several disadvantages. One disadvantage of the randomly dispersed glass beads 111 is that the glass beads 111 can stick together and can detrimentally affect image quality of the MIPD if they happen to land on the pixels of active area 106. Moreover, the random dispersal does not guarantee that the glass beads will provide the intended support.
A further disadvantage of typical fabrication methods concerns the epoxy barrier 120 used to contain the liquid crystal 110. The epoxy tends to spread out into some arbitrary shape as shown in the top view of FIG. 1B. This affects the distance between the cover glass 115 and the active area 106. This spreading also increases the dimensions of the substrate 105 necessary to fabricate the device because the spreading epoxy covers a significant portion of the substrate. This is particularly problematic in devices in which the epoxy bead is relatively large compared to the active area of the device. This can result in lower yield. This disadvantage has greater impact due to the small size of the MIPD as well as the proximity of the active area to an input/output area 125 used to connect the MIPD to a device package.